Transducer for producing sound at microwave frequencies



Dec. 11, 1956 J. c. SLATER 2,773,996

TRANSDUCER FOR PRODUCING SQUND AT MICROWAVE FREQUENCIES Filed Sept. 13, 1946 INV NT-0R JOHN CLARKE S ATER ATTORNEY nited States TRANSDUCER FOR PRODUCING SOUND AT MICROWAVE FREQUENCIES Application September 13, 1946, Serial No. 636,929

5 Claims. (Cl. 3108.1)

This invention relates in general to electroacoustical systems, and more particularly to electromechanical transducers capable of the generation or conversion of supersonic (compressional) waves of ultra high frequency.

Supersonic signals at ultra high frequencies are of considerable value in the study of the structure of fluids, solids, colloidal suspensions and the like. Such signals are also of assistance in obtaining information concerning electrical problems involving crystal detectors and delay lines; and may be applied to study various biological eflects.

The present invention contemplates and has as a primary object the provision of an electromechanical transducer operable at supersonic frequencies over a wide spectrum, including frequencies of the order of 3000 megacycles per second.

Another object of this invention is to provide a transducer in the form of a crystal or like piezoelectric element adapted for inclusion in high frequency electrical systems.

A further object of this invention is to provide a transducer, including a high frequency crystal operative within a resonant cavity.

A still further object of the present invention is to provide means for operating a high frequency crystal at a comparatively high harmonic of its fundamental frequency.

Another object of this invention is to provide a transducer readily adapted to supersonic experimentation.

These and other objects of the present invention will become apparent from the following specification when taken in connection with the accompanying drawing, the figure of which illustrates a preferred embodiment of an ultra high frequency transducer.

With reference now to the drawing there is illustrated an electrical system which comprises essentially a tunable resonant cavity and a transmission line coupled thereto. In the embodiment shown, the transmission line is a rectangular waveguide 11 having a terminating metallic plate 12. It will be understood that the particular type transmission line utilized is a function of the frequencies involved and need not be limited to the waveguide as shown. At frequencies of the order of 3000 megacycles per second a rectangular guide is well suited.

A tunable resonant cavity 13 is formed within a conductive cylindrical structure 14. At its lower end cylinder 14 is closed by a conductive wall 15, which is removably attached by cooperating threads 16 on both cylinder and wall. This removable arrangement permits inspection and adjustment of the oscillating system to be described below.

At its upper end the cylinder 14 is closed by a cap 17 having internal threads 21 for engagement with corresponding threads on the cylinder 14. A flat surface 22 is machined into the outer surface of cylinder 14 and is adapted to receive the end plate 12 of waveguide 11.

The cylinder 14 and the waveguide 11 are secured along surface 22 by soldering or the like, and similar aligned openings 23 are provided in the joined surfaces.- Openings 23 electrically couple resonant cavity 13 and transmission waveguide 11.

Positioned within cylinder 14 is a flexible, saucershaped diaphragm 24 which forms the upper electrical boundary surface of resonant cavity 13, and which is cricumferentially secured between a pair of cooperating circular flanges 25 and 26. The flanges 2S and 26 are in turn fixed to the inner surface of cylinder 14.

For tuning purposes, diaphragm 24 is engaged by a compression spring 27 which is fitted to the end of a knurled head bolt 31. The threads 32 of bolt 31 pass through a correspondingly threaded opening in the center of cap 17.

A conductive post 33 is centrally attached to the under surface of diaphragm 24, and extends into the cavity 13. The axial displacement of diaphragm 24 and post 33 may evidently be controlled by rotation of bolt 31.

A central opening 35 is provided in the cavity wall 15. A cylindrical supporting block 36 of quartz or similar substance is suitably secured within opening 35 such that the upper circular surface thereof is coplanar with the upper surface of wall 15. A piezoelectric element, such as disc-shaped quartz crystal 37 is employed as the supersonic transducer. This crystal is preferably polished and silvered, or otherwise metalized, over its lower surface. The upper surface of quartz block 36 is correspondingly silvered and both crystal and block are joined as shown in the drawing by soldering together these metalized surfaces. A low melting point solder, such as Woods metal is preferably employed, forming a conductive metallic film 41 at the junction, which film is in electrical contact with the surface of wall 15 along its circular edge.

The conductive film 41 functions, as a mechanical bond, and in addition, as a continuation of the metallic surface of the cavity wall 15, and permits the substantially unimpeded transfer of mechanical vibrations therethrough.

The upper surface of crystal 37 is separated by a small air gap from conductive post 33. When the cavity 13 is excited at its resonant frequency, as by radio frequency energy coupled from waveguide 11, an intense alternating electric field is established between post 33 and conductive film 41 through crystal 37. In order to obtain ultra high frequency supersonic waves, crystal 37 is excited at a harmonic considerably above its fundamental. For example, if transducer operation is desired at 3000 megacycles crystal 37 may have a 30 megacycle fundamental and be operated at its hundredth harmonic. Cavity 13 is designed to be electrically resonant at the harmonic frequency, or 3000 megacycles. Critical tuning for resonance of the system is accomplished by rotating bolt 31.

Supersonic energy developed by crystal 37 may be applied to samples under test by suitable mechanical coupling to quartz supporting block 36. For test of small liquid samples, the block 36 may be made in the form of a hollow cup and thus serve as a container.

As is generally known, a reciprocal relationship exists for piezoelectric or similar transducers; that is, the system functions equally well for the generation of supersonic waves from applied electrical signals, or for the generation of electrical energy from applied supersonic Waves. Accordingly, the apparatus illustrated may be used to convert supersonic vibrations applied to quartz supporting block 36 to electrical signals which may be transmitted through waveguide 11 to a suitable indicator. This permits the adaptation of the apparatus shown as a delay line for pulsed microwave signals. A radio frequency pulse transmitted in waveguide 11 energizes resonant cavity 13 whereby crystal 37 transmits a corresponding pulse of supersonic energy into block 36. This mechanical wave travels through block 36 and is partially reflected at the discontinuity represented by the end away from crystal 37. The reflected supersonic energy excites crystal 37, which in turn establishes an electric field in cavity 13, returning an electrical signal to the waveguide transmission system 11. The delay time between input and returned electrical pulses is essentially equal to the total travel time of supersonic energy through quartz block 36.

It will be noted that the particular construction employed readily permits the removal of wall 15 and its associated quartz block 36 and crystal element 37. This unit, or cartridge, may be inserted in electrical systems other than the resonant cavity shown, as for example, in the flat side of a waveguide. Upon the application of high intensity electrical fields of a multiple of the crystal resonant frequency, acoustical energy is developed and is coupled from the system through block 36. The metal film 41 provides a continuous current path under crystal 37 in whatever apparatus the cartridge is employed.

As various modifications and extensions of the principles of the present transducer may now, in view of the above disclosure become apparent to those skilled in the art, it is preferred that the spirit and scope of this invention be limited solely by the appended claims.

What is claimed is:

1. In combination, a resonant cavity, means for energizing said resonant cavity with electromagnetic energy, a piezoelectric crystal, said crystal having as an electrode a conductive film bonded to one of its surfaces, said crystal being secured to said resonant cavity such that said conductive film constitutes a portion of one of the end walls of said resonant cavity, a flexible diaphragm disposed across said cavity, a conductive post secured to one side of said diaphragm and projecting toward said crystal, said post serving as the other electrode of said crystal thereby increasing the electric field acting across said crystal upon energization of said resonant cavity, an output means attached to said conductive film for extracting supersonic energy from said piezoelectric crystal.

2. In combination, a resonant cavity, means for engendering a high frequency electromagnetic field within said cavity, a piezoelectric crystal, a conductive film bonded to one surface of said crystal, said conductive film constituting a portion of one of the end walls of said resonant cavity and serving as an electrode for said crystal, a flexible diaphragm extending across said resonant cavity, a conductive post attached to said diaphragm and extending toward said piezoelectric crystal, said post serving as the other electrode of said crystal whereby maximum electric field components of the electromagnetic field within said cavity are impressed across said crystal thereby causing the generation of high frequency supersonic waves, and means for axially displacing said diaphragm to tune said cavity to a harmonic of the fundamental frequency of said piezoelectric crystal, and means for coupling supersonic energy from said crystal.

3. In a combination as defined in claim 2 wherein said means for axially displacing said diaphragm within said cavity resonator comprises a spring loaded screw secured to the other end wall of said cavity resonator and bearing upon said diaphragm.

4. In a combination as defined in claim 2 wherein said means for coupling supersonic energy from said crystal comprises a semi-conductive block secured to said conductive film.

5. A transducer comprising a resonant cavity, means for exciting said cavity with electromagnetic energy, a semi-conductive block extending through an end wall of said resonant cavity and having a surface coplanar with the inner surface of said end wall, a piezoelectric crystal, a conductive film bonding a surface of said crystal to said block, said conductive film maintaining electrical continuity of said inner surface of said end wall, a diaphragm extending across said resonant cavity opposite said piezoelectric crystal, a conductive projection attached to one side of said diaphragm and extending towards said piezoelectric crystal, a spring loaded screw bearing upon the other side of said diaphragm for tuning said cavity resonator, said projection serving to increase the electric field component of said electromagnetic energy acting across said piezoelectric crystal.

References Cited in the file of this patent UNITED STATES PATENTS 2,174,701 Kock Oct. 3, 1939 2,217,280 Kock Oct. 8, 1940 2,280,226 Firestone Apr. 21, 1942 2,404,226 Gurewitsch July 16, 1946 2,409,321 Shephan Oct. 15, 1946 2,413,939 Benware Jan. 7, 1947 2,414,456 Edson Jan. 21, 1947 2,427,100 Kihn Sept. 9, 1947 2,463,472 Boch Mar. 1, 1949 OTHER REFERENCES Cady: Piezoelectricity, McGraw-Hill, New York, 1946, pages 678-681.

Cady: Piezoelectricity, McGraw-Hill, New York,

1946, page 682. 

